This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application for APPARATUS FOR FABRICATING AN OPTICAL FIBER AND METHOD THEREFOR earlier filed in the Korean Industrial Property Office on the 10th day of June 1996 and there duly assigned Ser. No. 20635/1996, a copy of which application is annexed hereto.
1. Field of the Invention
The present invention concerns an apparatus for drawing an optical fiber from an optical fiber preform. In particular, this invention concerns an apparatus for simultaneously fusing a core preform and a cladding tube and drawing a clad optical fiber from the fused combination.
2. Description of the Related Art
Quartz optical fibers, such the fibers currently used in ultra high speed data communication networks, typically are fabricated by one of two methods: the outside vapor deposition (OVD) method or the modified chemical vapor deposition (MCVD) method. The OVD method comprises the steps of hydrolyzing a chemical gas consisting of gaseous SiCl4 and a dopant by firing with a simultaneously supplied oxygen to deposit SiO2 soot on the outside of a suitable starter rod: and dehydrating the deposited soot in a high temperature furnace by using Cl2 and He, which also sinters the soot so as to form an optical fiber preform.
The MCVD method requires more complicated processing but has certain advantages over OVD. MCVD involves deposition of several layers on the inside of a quartz tube by simultaneously supplying oxygen and a chemical gas consisting of SiCl4 and a dopant. In the layer deposition process the cladding layers are laid down first, and then the layers that will form the core are deposited. After deposition of the layers, the internally-layered quartz tube is heated in the presence of Cl2 and He so as to form a compact quartz rod.
The MCVD method, used by itself, has the inherent limitation that it is not suitable to make preforms of more than 25 mm in diameter. In order to overcome this limitation, MCVD is often practiced with a so-called overcladding method, which allows fabrication of relatively large preforms and thus improves productivity for the fiber fabrication process. Overcladding involves, in general terms, placing a rod preform inside a tube made of a suitable overcladding material, fusing the rod and tube together to form a secondary preform, and drawing from the secondary preform an optical fiber comprising a core enclosed within a cladding layer. In this form of MCVD, obviously, the layers generated in the internal deposition process need not include both core layers and cladding layers.
Thus, a high-productivity implementation of the MCVD method requires three essential steps: preparing a primary optical fiber preform by internal deposition, overcladding the primary optical fiber preform to obtain a secondary optical fiber preform, and finally drawing an optical fiber from the secondary optical fiber preform. Carrying out these three steps separately requires substantial amounts of time and consequently has a negative effect on productivity. Another disadvantage of this approach is that the step of overcladding the primary optical fiber preform typically requires a large amount of oxygen or hydrogen. Furthermore, for the case where the primary optical fiber preform is relatively large, the overcladding step in itself requires application of a relatively large amount of heat. This application of heat, in combination with the effects of the heat necessary for the drawing step, has the effect of degrading the transmission characteristics, such as attenuation characteristics, of the finished optical fiber.
Combination of the overcladding and drawing steps has long been a goal in the glass fiber industry. See, for example, U.S. Pat. No. 2,980,957, issued in 1961 to Hicks, Jr., and U.S. Pat. No. 3,037,241, issued in 1962 to Bazinet, Jr. et al. These early attempts recognized the advantage of using a partial vacuum, or pressure differential between the outside and the inside of the tube, to promote collapse of the overcladding tube about the core rod prior to a drawing stage. They could not succeed, though, in consistently producing optical fibers with the long lengths and low loss characteristics necessary for modern fiberoptic communications applications. As a result, the industry has substantially relied upon deposition methods and rod-and-tube methods including a separate preform fabrication stage.
One problem inherent in combining the fusing and drawing stages has been to control the application of vacuum with sufficient precision that the finished optical fiber has sufficient strength and optical quality for modern communications applications. U.S. Pat. No. 4,772,303, issued to Kamiya et al., for example, proposed an MCVD technique combining drawing and tube collapsing in a continuous process. This process comprises application of a partial vacuum, but it requires a sophisticated suction system including a vacuum pump, a differential pressure gauge, an inert gas supply, a gas flow rate regulator, a flow rate setter, and preferably an automatic switching valve. Such a system provides for a small pressure differential and allows fine adjustment if it, but it is expensive and complicated to implement. Also, because it does not include overcladding, this approach does not address the issue of proper alignment between rod and tube.
Proper alignment goes to another intrinsic problem in overcladding methods: controlling the thickness of the cladding layer on the optical fiber produced to ensure uniformity. For example, U.S. Pat. No. 4,820,322, issued to Baumgart et al., presented a rod-and-tube approach that yields a strong fiber with concentric core and cladding. This method uses vacuum to promote collapse of the overcladding tube, and it can be practiced either in a separate manufacturing phase or in a continuous process combined with drawing of the fiber. However, it lacks any particular means to control the vacuum applied and thus may require complicated vacuum equipment to achieve optimal results. The Baumgart approach also has a limit on the gap between the rod and the overcladding tube: the tube inside diameter cannot exceed the rod diameter by more than a certain amount. Furthermore, the embodiment combining collapsing the tube and drawing the fiber does not use an affirmative means to center the rod in the tube, relying instead for concentricity on inherent self-centering forces thought to be present as the clad fiber is drawn from the tip of the rod-and-tube preform.
We have therefore noticed that a need exists for a simple apparatus and method of drawing a high quality optical fiber from a rod-and-tube preform while simultaneously fusing the rod and the overcladding tube. This approach should employ a low-intensity vacuum source that permits fine adjustment of the differential pressure. It should also provide for controlled alignment of the core rod and the overcladding tube to ensure that the desired circumferential uniformity of the cladding layer in the drawn fiber is achieved. To maximize productivity, though, the process should not require an intricate alignment procedure as the rod and tube are mounted in the drawing plant. Preferably, these objectives would be achieved through an inexpensive apparatus that requires little hardware in addition to the equipment already present in the drawing plant.
It is therefore an object of the present invention to provide an apparatus and a method for overcladding a primary optical fiber preform while simultaneously drawing a finished optical fiber from the overclad preform.
It is also an object of this invention to include in such an apparatus and method a means to apply a finely-controlled pressure differential to the operation of collapsing the overcladding tube onto the core preform.
It is another object of this invention to provide such an apparatus and method that will reduce the production costs for fabricating optical fibers while maintaining or improving the quality of the finished fibers, such as their low attenuation characteristics.
It is a further object of the present invention to provide such an apparatus and method capable of being implemented with inexpensive additional hardware.
To achieve these and other objects, the present invention provides in one aspect an apparatus for fabricating an optical fiber from a primary optical fiber preform and an overcladding tube, the apparatus comprising an adjoiner, a furnace, and a preform positioner. The primary optical fiber preform has a first primary axis and an outer surface, and the overcladding tube has a second primary axis and an inner surface and defines within it an interior space. The adjoiner is adapted to be assembled with the primary optical fiber preform and the overcladding tube into a secondary preform assembly. The adjoiner is further adapted to hold, in the secondary preform assembly, the primary optical fiber preform in a centrally inserted position within the interior space defined by the overcladding tube and with the first and second primary axes in substantial alignment. The adjoiner has defined within it a plurality of passages that provide an inlet for receiving a flow of gas, a region in which the flow of gas generates a condition of reduced pressure, and an extension of the region of reduced pressure to the interior space. The furnace heats a portion of the primary optical fiber preform and a portion of the overcladding tube to a softened state from which an optical fiber can be drawn. The preform positioner positions the secondary preform assembly in a specified position with respect to the furnace.
A second aspect of the present invention provides a method for fabricating an optical fiber from a primary optical fiber preform and an overcladding tube. The primary optical fiber preform and the overcladding tube are similar to those associated with the first aspect of the invention. The method includes the steps of assembling a secondary preform assembly having an upper end and a lower end and including the primary optical fiber preform, the overcladding tube, and an adjoiner. The adjoiner holds the primary optical fiber preform in a centrally inserted position within the interior space of the overcladding tube and with the first and second primary axes in substantial alignment. The adjoiner also has defined within it a plurality of passages to provide an inlet for a flow of gas, a region in which the flow of gas generates a condition of reduced pressure, and an extension of the region of reduced pressure to the interior space. The method further includes the step of sealing the lower end by heating to form a sealed preform assembly and applying a lower portion of the sealed preform assembly to a furnace and applying a flow of gas to the inlet of the adjoiner, thereby fusing a part of the overcladding tube adjacent to the lower end of the sealed preform assembly to the primary optical fiber preform. The method also includes the step, simultaneous with the applying step, of drawing from the lower end of the sealed preform assembly an optical fiber.